Final answer:
Christian attitudes toward war changed from initial non-violence to sanctioning violence in the name of God by the 4th century AD. The Crusades institutionalized the concept of holy war, and chivalry sought to regulate knightly conduct in war. However, modern secularization led Western societies to reject religious warfare.
Step-by-step explanation:
Christian attitudes toward war underwent significant changes over time, particularly from non-violent beginnings to accepting and institutionalizing warfare in certain contexts. The shift began in the 4th century AD, where cultural developments overrode the earlier teachings of Jesus Christ, leading to a period where large-scale violence was deemed acceptable if done on the side of God. By the time of the Crusades, the concept of holy war became deeply entrenched, with warfare on behalf of God being seen as a 'just war,' especially if called by the pope.
In the Middle Ages, chivalry and the ecclesiastical endorsement created a framework that distinguished between legitimate and illegitimate warfare, causing knights to adhere to certain codes of conduct. However, with the rise of nation-states and secularization, modern Western societies largely reject warfare for religious reasons, reflecting a significant shift from the perspectives that justified the Crusades.
The East-West Schism and political complexities within the Christian Church, including the influence of secular rulers, also played roles in shaping attitudes toward war. Gradually, as Christianity spread, sometimes through force, European society became almost uniformly Christian by the eleventh century, influencing perspectives on warfare and conversion.